When a head in a disc drive is moved by its corresponding servo-controlled actuator from one track to another selected track, the servo system that controls the actuator must closely monitor and control the velocity of the head on a real time basis. For this purpose, the position information is derived from the read signal from the head.
The analog read signal from the head is converted to a digital read signal by an analog to digital (A/D) converter. The servo system then derives position information from the digital read signal.
For a sampled data control system disc drive, embedded servo position information in the read signal is periodically sampled and passed to a microprocessor. The microprocessor calculates the measured velocity V.sub.m by taking the difference between the position indicated by the current sample and the position indicated by the previous sample and dividing the difference by the sample period.
The microprocessor generates an acceleration command that is used to correct any difference between the measured velocity of the head V.sub.m and a desired velocity of the head V.sub.d according to a seek velocity profile that is appropriate given the number of tracks left to travel to the destination track. The microprocessor command is converted to a constant current analog control signal by a digital to analog converter. This control signal drives the actuator and thereby corrects the velocity of the head.
The seek velocity profile that is used by the microprocessor to determine what level of current to command to the actuator is implemented with the process of comparing the present track position of the head with the selected track position and selecting the desired velocity V.sub.d based upon this difference in tracks. Typically the microprocessor has a table of desired velocities based upon the difference in tracks for this purpose.
The table of desired velocities V.sub.d that are stored in the microprocessor are generally based upon empirical evaluation of a sample of disc drives operating under worst case conditions with substandard actuator motors. As a result, drives with greater seek performance potential operating under normal conditions are not able to seek any faster than weak drives operating under unfavorable conditions.